Researchers develop brain-powered prosthetic limbs for amputees

People with amputated legs have become able to control their prosthetic limbs with their brains, a significant scientific advancement that has made their gait more effortless and enhanced their ability to overcome obstacles, according to a study. Study This report was published in the journal Nature Medicine on Monday.

Researchers at the K. Lisa Yang Center for Bionics at the Massachusetts Institute of Technology and Brigham and Women’s Hospital have paved the way for the next generation of prosthetic limbs by creating a connection between a person’s nervous system and his or her prosthetic leg.

“We were able to show, for the first time, full neural control of bionic walking,” said Hyunghun Song, first author of the study and a postdoctoral researcher at MIT.

Most cutting-edge bionic prostheses rely on pre-programmed robotic commands rather than the user’s brain signals. Advanced robotic technologies can sense the environment and repeatedly activate predefined leg movements to help the person navigate that kind of terrain.

But many of these robotics work best on flat ground and struggle to overcome common obstacles like bumps or puddles. The person wearing the prosthetic often has little power to adjust the prosthetic once it is in motion, especially in response to sudden terrain changes.

“When I walk, it feels like somebody is driving me because an algorithm is sending the commands to the motors, and I’m not doing it,” said Hugh Herr, the study’s principal investigator and a professor of media arts and sciences at MIT and a pioneer in biomechatronics, a field that combines biology with electronics and mechanics. Herr’s legs were amputated below the knee due to frostbite several years ago, and he uses advanced robotic prostheses.

“There is growing evidence that [showing] “When you connect the brain to a mechatronic prosthesis, an embodiment occurs, where the person views the prosthesis as a natural extension of their body,” Herr said.

The authors worked with 14 study participants, half of whom were recommended below-knee amputations via a method called the agonist-antagonist myoneural interface (AMI), while the other half were recommended traditional amputations.

“What’s great about this is that it’s leveraging technological innovation as well as surgical innovation,” said Conor Walsh, a professor at the Harvard School of Engineering and Applied Sciences who specializes in the development of wearable assistive robots and was not involved in the study.

AMI amputation was developed to address the limitations of traditional leg amputation surgery, which cuts vital muscle connections at the amputation site.

Movements are made possible by the way muscles move at joints. One muscle – known as the agonist – contracts to move the limb and another – known as the antagonist – lengthens in response. For example, during a biceps curl, the biceps muscle is the agonist because it contracts to raise the forearm, while the triceps muscle is the antagonist because it lengthens to enable movement.

When muscle attachments are surgically severed, the patient’s ability to feel muscle contractions is impaired after surgery, and as a result, their ability to accurately and minutely feel where their prosthesis is in space is affected.

In contrast, the AMI procedure reconnects muscles to the remaining limb, thereby mimicking the valuable muscular feedback one receives from the intact limb.

Eric Rombaucas, an assistant professor of mechanical engineering at the University of Washington who was not involved in the study, said the study is part of a movement toward “next-generation prosthetic limb technologies that focus not just on motion but also on sensation.”

The AMI procedure for below-knee amputation was named Ewing amputation after Jim Ewing, who was the first person to receive the procedure in 2016.

Patients who underwent Ewing amputation had less muscle atrophy in the remaining limb and less phantom pain, i.e. the experience of discomfort in the limb that is no longer there.

The researchers fitted all participants with a new bionic limb, consisting of an artificial ankle that measures electrical activity from muscle movement and electrodes placed on the surface of the skin.

The brain sends electrical impulses to the muscles, causing them to contract. The contractions generate their own electrical signals, which are detected by electrodes and sent to small computers mounted on the prosthetic limb. The computers then convert those electrical signals into force and movement for the prosthetic limb.

Study participant Amy Pietraffitta, who was given the Ewing amputation after suffering severe burn injuries, said the bionic limb gave her the ability to bend both of her legs and dance again.

“Having that kind of flexibility made it so much more real,” Pietraffitta said. “It felt like everything was right there.”

With their enhanced muscle sensations, participants who had Ewing amputations were able to walk faster and with a more natural gait using their bionic limbs than those who had traditional amputations.

When a person has to deviate from their normal walking pattern, they usually have to work harder to move around.

“That expenditure of energy … forces our heart to work harder and forces our lungs to work harder … and that can slowly destroy our hip joints or our lower spine,” said Matthew J. Carty, a reconstructive plastic surgeon at Brigham and Women’s Hospital and the first doctor to perform the AMI procedure.

Patients who received the Ewing amputation and the new prosthesis were able to climb ramps and stairs with ease. They easily adjusted their legs to climb stairs and also absorbed the shock when descending.

The researchers hope that this innovative prosthesis will become commercially available in the next five years.

“We’re starting to get a glimpse of this fantastic future where a person can lose a large portion of their body, and have the technology available to reconstruct that part of their body with full functionality,” Herr said.

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